Liquid crystal display devices have been applied to, for example, watches, calculators, a variety of measuring equipment, panels used in automobiles, word processors, electronic notebooks, printers, computers, television sets, clocks, and advertising boards. Representative examples of types of liquid crystal display devices include a TN (twisted nematic) type, an STN (super twisted nematic) type, and a vertical alignment type and IPS (in-plane switching) type in which a TFT (thin film transistor) is used. Liquid crystal compositions used for such liquid crystal display devices need to satisfy the following requirements: being stable to external elements such as moisture, air, heat, and light; exhibiting a liquid crystal phase (nematic phase, smectic phase, or blue phase) in a wide temperature range mainly including room temperature as much as possible; having a low viscosity; and enabling a low driving voltage. In addition, liquid crystal compositions need to have, for example, dielectric anisotropy (Δ∈) and refractive index anisotropy (Δn) optimum to individual display devices.
A liquid crystal composition having positive Δ∈ is used in horizontal alignment-type displays such as a TN type, an STN type, and an IPS type. There has been a report on another type of driving; in particular, molecules of a liquid crystal composition having positive Δ∈ are vertically aligned in a state in which voltage is not applied, and then a horizontal electric field is applied for performing display. A demand for a liquid crystal composition having positive Δ∈ has therefore further increased. In all types of driving, however, there have been demands for improvement of response speed, and a liquid crystal composition having a lower viscosity than typical liquid crystal compositions is needed to satisfy such demands. In order to develop the liquid crystal composition having a low viscosity, it is effective to decrease the viscosity of individual compounds contained in the liquid crystal composition. In the case where a liquid crystal composition is applied to, for example, display devices, the liquid crystal composition needs to exhibit a liquid crystal phase stable in a wide temperature range.
In general, in terms of production of a compound having a low viscosity, it has been believed that the compound preferably has a molecular framework in which multiple cyclic structures are directly bonded to each other via no linking group, namely a structure called directly connected rings. Compounds having positive Δ∈ and a structure in which three or more rings are directly connected to each other are generally highly crystalline; in the case where a liquid crystal composition containing such a compound is stored for a long time, the crystals of this compound precipitate, which is problematic. In order to improve the storage stability of such a compound, a variety of compounds into which linking groups are introduced have been studied. Although the introduction of linking groups increases viscosity to some extent, miscibility in a liquid crystal composition can be enhanced, and the precipitation of crystals can be reduced (see Patent Literatures 1 to 8). A compound having a —CH2O— group as a linking group is highly chemically stable, and a liquid crystal composition containing such a compound exhibits a liquid crystal phase stable in a wide temperature range; however, the viscosity of this liquid crystal composition is extraordinarily high, which is problematic.
In order to produce a liquid crystal composition that exhibits a stable liquid crystal phase in a wide temperature range, a composition having a high clearing point (T−i) is effectively used. In general, increasing the number of ring structures contained in a compound is effective to produce a compound having a high T−i. Increasing the number of ring structures contained in a compound, however, leads to an enhancement in crystallinity with the result that crystals are likely to precipitate from the liquid crystal composition, which is problematic.